Polyimides having high Tg, high TOS, and low moisture regain

ABSTRACT

Polyimide polymers from 3,4,3&#39;,4&#39;-biphenyltetracarboxylic dianhydride and 3,4,3&#39;,4&#39;-benzophenonetetracarboxylic dianhydride and a diamine such as p-phenylenediamine exhibit a high glass transition temperature, high thermal oxidative stability and low moisture regain, useful for structural applications.

This application claims benefit of U.S. Provisional Application SerialNo.60/031,583 filed Dec. 5, 1996 and U.S. Provisional Application SerialNo. 60/038,938 filed Feb. 24, 1997.

FIELD OF THE INVENTION

The current invention relates to polyimide polymers characterized byhigh glass transition temperatures, high thermal oxidative stabilities,and low moisture regain. The current invention also relates to theprocess of making these polyimides and their use in applications such ascomposites, films, laminates, and other products.

BACKGROUND OF THE INVENTION

Polymers comprised of polyimides have been found to have a variety ofdesirable qualities that are useful in high temperature applications. Ingeneral, and particularly for use as matrix materials in structuralcomposites, the desirable qualities include a high glass transitiontemperature (Tg), high thermal oxidative stability (TOS), low moistureregain, and low conversion costs. While polyimide polymers exhibitingone or some of these qualities are known, there is a need for polyimidepolymers that have all of these advantages.

Known composite systems having desirable characteristics include systemsdesignated as Avimid® K and Avimid® N (E.I. du Pont de Nemours and Co.).

Avimid® K, described in A. R. Wedgewood, SAMPE Tech. Conf. 24, p. T385,1992, employs a matrix based on pyromellitic dianhydride and extendedether diamines. These polymers are compatible with composite moldingprocesses featuring melt consolidation of devolatilized prepreg plies.The glass transition temperature range of the polyimide component ofAvimid® K is in the range of from 220 to 255° C.

Avimid® N utilizes a polyimide polymer prepared from2,2'-bis(3,4-dicarboxyphenyl)-hexafluoropropane tetracarboxylic acid(6FTA), the dianhydride form of which is referred to as 6FDA, along witha mix of p-phenylenediamine (PPD) and m-phenylenediamine (MPD). Thispolyimide polymer, designated and known in the art as NR-150, has aglass transition temperature range of about equal to or greater than340° C., with a reported moisture regain of about 3.7% by weight. See R.J. Boyce and T. P. Gannett, HIGH TEMPLE WORKSHOP XV, N, January 1995.

Another known polyimide polymer is comprised of biphenyl dianhydride andoptionally pyromellitic dianhydride (PMDA) with1,4-bis(4-aminophenoxy)-2-phenylbenzene (2PhAPB144) and MPD or PPD. SeeU.S. Pat. No. 5,478,913. This polymer exhibits a glass transitiontemperature range of about 220 to about 330° C., with a moisture regainreported in R. J. Boyce and T. P. Gannett, HIGH TEMPLE WORKSHOP XV, N,January 1995, of about 2.8%.

The current invention provides novel polyimides which have relativelyhigh Tg and with thermal oxidative stability and moisture regaincharacteristics significantly better than those exhibited by 6FTA or6FDA based resins. The polyimide polymers of the current invention havegood mechanical properties, making them useful in a wide variety ofapplications. Moreover, the invention has the potential of cost savingsover the polyimides of the state of the art because the acid functionalcomponents of the current invention are relatively less expensive thanthe acid functional components of known polyimide polymers.

SUMMARY OF THE INVENTION

The current invention provides novel polyimide polymers having a highglass transition temperature (Tg), high thermal oxidative stability(TOS), and very low moisture regain, comprising the following recurringstructural units: ##STR1## wherein the mole ratio of BTDA to BPDA in thepolymer is from more than about 30 to less than about 70 (>30:<70) toabout 80 to 20 (80:20), and Q is a suitable aromatic moiety. Alsoprovided by the current invention are such polyimides formed from aprecursor solution having a stoichiometric excess of either the diamineor dianhydride (or derivatives thereof) components as a means ofcontrolling the molecular weight of the polyimide polymers formed.Moreover, the current invention also includes the use of end-cappingagents. The end-capping agents may be non-reactive, reactive (if, forexample, cross-linking is desired), or a mixture of both.

In addition to the resulting polyimides themselves, the currentinvention also encompasses the resins which can be formed therefrom, andcomposites and prepregs reinforced with substrates. The substrates maybe any of those known in the art, including but not limited to materialssuch as glass, carbon, metallic, and aramid fibers or particles. See,e.g., U.S. Pat. No. 5,138,028.

The invention likewise encompasses, but is not limited to, the use ofthe polyimides of the current invention in applications such as filmsand laminates.

DETAILED DESCRIPTION OF THE INVENTION

The polyimides of the current invention are prepared by reacting3,4,3',4'-biphenyltetracarboxylic dianhydride (biphenyl dianhydride orBPDA) and 3,4,3',4'-benzophenone-tetracarboxylic dianhydride (BTDA) witha suitable diamine component, or a mixture of suitable diaminecomponents, such as p-phenylenediamine (PPD) and m-phenylenediamine(MPD). BTDA and BPDA are acid functional components of the polyimides ofthe current invention. Reference to BTDA, BPDA, or acid functionalcomponents is intended to include all of the functional equivalent formsthereof, that is, such reference includes but is not limited to thedianhydride, tetracarboxylic acid, ester, or diester-diacid forms, othercommon derivatives known in the art, or any mixtures thereof. Thespecific acid functional components of the current invention may beinitially added as any of the forms or equivalents as defined. Areactive or non-reactive end-capping agent can be employed if desired,for example, as disclosed in U.S. Pat. No. 5,478,913.

Polymerization according to the current invention is accomplished byadding BPDA and BTDA in the appropriate ratios to a solution ofN-methylpyrrolidone (NHP) and ethanol, forming the diethylester diacid,and then adding the diamine component.

The diamine or diamine mixture is chosen from the group comprisingaromatic diamines, such as a mixture of p-phenylenediamine (PPD) andm-phenylenediamine (MPD), PPD alone, or other diamines chosen toenhance, or at least not adversely affect, the desired characteristicsachieved by use of the disclosed ratio of BTDA to BPDA in the acidfunctional component of the polyimide. The desired polyimide of thecurrent invention is formed by the subsequent heating, and removal ofthe volatiles. The resulting material can be ground and formed into adesired shape under heat and pressure. The precursor solution can alsobe used to form composites by impregnating a fibrous reinforcingsubstrate with the precursor solution, or to form other structures bymixing the precursor solution with other types of substrates. In thecase of using fibrous substrates for the creation of composites, theimpregnated substrate is heated to form the polyimide, and the resultingmaterial can then be shaped using heat and pressure.

Contrary to what might be expected from the nature of the components ofthe precursor solution, the resulting polyimides of the currentinvention exhibit excellent Tg, TOS, and moisture regain characteristicswhen compared to previously known polyimides, and are formed of readilyavailable, economical ingredients.

The mixture of the acid functional components of the resultingpolyimides of the current invention should be such that the mole ratioof BTDA to BPDA is within a defined range. The BTDA/BPDA mixture,whether comprising the entirety of the acid functional component or amajor portion of the acid functional component of the polyimides, shouldbe comprised of a mixture wherein the amount of BTDA should be higherthan about 30 mole percent of the BTDA/BPDA mixture, up to about 80 molepercent of BTDA, while the amount of BPDA should be in the range of fromless than about 70 mole percent to about 20 mole percent. Excellentresults are achieved where BTDA constitutes about 70 mole percent of theBTDA/BPDA mixture. It is within the scope of the current invention toadd minor amounts of other acid functional components to the mixture ofBTDA and BPDA, so long as the ratios of BTDA to BPDA measured withrespect to each other are as disclosed herein. For example, addition ofminor amounts of other acid functional groups may be desired to maximizeother characteristics of the resulting polyimide polymer, but theaddition of the acid functional component or components other than BTDAand BPDA must be limited such that the concentration of the added acidfunctional components does not adversely affect the Tg, TOS, andmoisture regain characteristics exhibited by polyimides made asdisclosed herein.

The current invention can be characterized as being a polyimide having aglass transition temperature greater than about 300 degrees Centigradecomprising structural units derived from3,4,3',4'-benzophenonetetracarboxylic dianhydride (BTDA),3,4,3',4'-biphenyltetracarboxylic dianhydride, and one or more aromaticdiamines wherein the mole ratio of the structural units derived fromBTDA to the structural units derived from BPDA is more than about 3/7 upto about 4.0. A more preferred form of the polyimide polymer is thepolymer wherein BTDA comprises from about 60 to about 80 mole percent ofthe BTDA/BPDA acid functional component, with the amount of BPDA rangingfrom about 20 to about 40 mole percent of the BTDA/BPDA component. Inthe most preferred form, the ratio of BTDA to BPDA is about 70 molepercent BTDA to about 30 mole percent BPDA. A preferred diaminecomponent for forming the polyimides of the current invention iscomprised of PPD in the range of from about 95 to 100 mole percent ofthe diamine component, and MPD in the range of up to about 5 molepercent of the diamine component, with a preferred ratio of about 95mole percent PPD and about 5 mole percent MPD. In its most preferredform, the resulting polyimide will have a Tg of about 338° C., a thermaloxidative stability in terms of percent weight loss of about 4.3percent, and a moisture regain characteristic in terms of percent weightgain of about 1.1 percent, all these characteristics determined andmeasured as described herein.

The diamine component used in forming polyimides of the currentinvention is chosen from the group comprising aromatic diamines or otherdiamines chosen to enhance, or at least not adversely affect, thedesired characteristics achieved by use of the disclosed ratio of BTDAto BPDA in the acid functional component of the polyimide. A suitablediamine component for use in forming the polyimide polymers of thecurrent invention is a mixture of p-phenylenediamine (PPD) andm-phenylenediamine (MPD). When this mixture of diamine components isused to form the polyimides, the diamine mixture should be such that PPDconstitutes from about 95 to 100 mole percent of the diamine component,with MPD constituting from about 0 to 5 mole percent. Another suitablediamine component, used alone or as part of a mixture, is1,4-bis(4-aminophenoxy)-2-phenylbenzene (2PhAPB144). As with the acidfunctional components, the functional equivalents of the diamines may beused, as is known in the art. The specific diamines chosen as thediamine components of the polyimides of the current invention should bechosen so as to enhance, or at least not adversely affect, the desiredTg, TOS, and moisture regain characteristics achieved by use of thedisclosed BTDA/BPDA ratios.

Either the diamine mixture or the acid functional mixture may be addedin amounts calculated to provide slight stoichiometric excess over theother component. Generally, the excess is chosen to be not more thanabout 40 mole percent more than the amount of the acid or diaminecomponent needed for an equimolar reaction of acid and diamine.

End-capping agents can be used as is known in the art to controlcharacteristics of the polyimide such as the molecular weight of theresulting polyimide polymers, to provide cross-linking capability, orboth. Either reactive or non-reactive end-capping agents, or a selectedmixture of both, may be used. Phthalic anhydride or phenyl amine, forexample, can be used as non-reactive end-capping agents. Suitablereactive end-capping agents include, for example, phenylethynylaniline(PEA) and phenylethynyl phthalic anhydride (PEPA). The use of a reactiveend-capping agent enables cross-linking to take place when the polyimideis heated during the curing process. The end-capping agents, whetherreactive or non-reactive, can be introduced with either the diamine oracid functional components.

Tests and Measurements

The glass transition temperatures (Tg) of the resins in the followingexamples were determined by forming a resin plaque, that is, a plaquemade of only the polyimide polymer, of the resin to be tested. A DynamicMechanical Analysis (DMA) was performed by plotting storage modulusvalues against temperature. Tangents to the curve before and after theinflection point were drawn and the temperature value of theintersection of these tangents was used as the Tg value. Measurementswere taken using a TA Instruments 9900 system with a heat-up rate of 10°C. per minute in air with a maximum temperature of 500° C.

Thermal oxidative stability (TOS) values for the resins in the followingexamples were determined as follows. Resins were formed into 3"×6"(0.176 m×0.152 m) resin plaques. Coupons measuring 1"×1" (0.025 m×0.025m) were cut from the plaques and dried at 120° C. for sixteen hours. Thedry weight of the coupons was measured. The coupons were then exposed toflowing air at 750° F. (399° C.) for one hundred hours in a Grieve Class"A" oven. The weight of the samples was then determined and weightlosses were calculated in weight percent. Samples were usually run induplicates from which average values were calculated.

Moisture regain for the resins in the examples were measured as follows.coupons of resin plaques were prepared as described above for the TOSmeasurements. Duplicate coupons were placed in a humidity chambermaintained at 60° C. and 95% relative humidity (R.H.). Weight gains asweight percent were measured on a regular time basis until saturationwas achieved. The humidity chamber used was a Blue M Humid-Flowcombination temperature and humidity cabinet.

EXAMPLES

Preparation of Polyimide Solution

Various polyimides were prepared in accordance with the currentinvention. All of the polyimides of the current invention were made inthe same general way, with additional polyimides being prepared toillustrate and compare the characteristics of the polyimides made inaccordance with the current invention. The relative proportions of theBTDA and BPDA were varied to produce polyimides from precursor solutionsin which the ratio of BTDA was varied in increments of from about 20 toabout 80 mole percent with respect to the proportion of BPDA, while theamount of BPDA used was accordingly varied in increments of from about20 to about 80 mole percent with respect to the proportion of BTDA. Allof the examples shown here were made with the diamine componentcomprising a mixture of about 95 mole percent PPD and about 5 molepercent MPD. For comparison purposes, a sample of NR-150 resin wasprepared by the method described herein. This resin was prepared using100 mole percent of 6FTA as the acid functional or dianhydridecomponent, while the diamine component was the same mixture (PPD:MPD: :95:5) used in preparation of the polyimides of the current invention.The particular amounts used are set forth in the tables.

The samples were prepared as follows. A glass reaction vessel, forexample, a 500 ml. 4-necked round bottomed flask, was fitted with awater condenser having an N₂ purge set-up at the top. An agitator and athermocouple were fitted to the vessel also. The vessel was thoroughlypurged with nitrogen. To the vessel were added 128 grams ofN-methylpyrrolidone (NMP) and 128 grams of anhydrous ethanol (E). Thissolvent mixture was then heated to 30° C. BTDA and BPDA as either thedianhydrides or diacids were added, and the solution was agitated whilethe solution was raised to a temperature of 90+/-2° C. and held for twohours to effect complete dissolution. With continuing agitation, thesolution was then cooled to 75+/-2° C. The diamine components were thenadded to the vessel, and this solution was held for 1.5 hours withcontinued agitation. The solution thus formed was then transferred to acontainer and stored until needed. In this form, the solution is knownin the art as a precursor or binder solution. It may be used toimpregnate various substrates, such as fibers, to form prepregs, or maybe further processed as in its pure, or neat, form for other uses.

Isolation of Polyimide Neat Resin Powder

The polyimide resins used in the plaques and coupons from which the datain the tables were derived were prepared as follows. Approximately 100grams of the sample solution prepared as described above was placed intoa 2" deep aluminum pan. The pan with the solution was then placed into avacuum oven, which was maintained at a temperature of 110° C. and apressure of 10" (0.25m) Hg. A dry nitrogen bleed was maintainedthroughout the entire process. The sample was held at 110° C. for aboutone hour or until the flow of volatilized solvent to the oven'scondenser had substantially stopped. The oven temperature was thenraised to 200° C. and held there for one hour.

The resulting partially devolatilized and partially cured material wasthen removed from the oven, cooled in a dessicator, and then ground in aWaring type blender. The resulting powder was then placed in an aluminumpan, and heated in a Class A oven and held at 330° C. (625° F.) for twohours. The material was then removed from the oven, cooled in adessicator, and ground into a fine powder. The powder was placed in anairtight container for storing.

Formation of Resin Plaques

The neat resin plaques from which the sample coupons were cut wereformed by utilizing a compression press. The press was preheated to 490°C. A mold measuring 3"×6" (0.176 m×0.152 m) was placed on the pressplaten, and 60 grams of the selected resin powder was placed in the moldand held with minimal pressure. When the temperature of the mold in thepress reached 430° C., the pressure on the mold was gradually increasedover a period of two minutes to a final pressure of 3,300 pounds persquare inch (224.55 atm.) and then held at this value. When thetemperature of the mold reached 480° C., the press was allowed to coolto about 300° C. After cooling, the pressure was released, the mold wasremoved from the press, and the neat resin plaque was removed from themold and subjected to the tests as described above.

Table 1 shows the components used in the foregoing procedures forpreparing a precursor solution in which the ratio of BTDA to BPDA was 70to 30. In this example, the acid functional component was comprised ofonly BTDA and BPDA, and the ratio was therefore 70 mole percent BTDA and30 percent BPDA. This acid functional component was mixed with a diaminesolution of 95 mole percent PPD and 5 mole percent MPD in the NMP andethanol solvent. The table shows the ingredients, the molar amounts ofBTDA, BPDA, PPD, and MPD used, and the respective weights in grams ofall the ingredients used to produce the precursor solution.

Table 2 sets forth the mole percent amounts and equivalent weights usedin preparing samples in which the ratio of BTDA was varied from about 20to about 80, and the corresponding ratio of BPDA was varied respectivelyfrom about 80 to about 20. For these samples, BTDA and BPDA comprisedthe acid functional component, so that the ratios are shown as molepercent ratios. The table also shows the use of 100 mole percent and theequivalent weight of 6FTA used to prepare the resin designated as NR-150used for comparison. For each mixture shown in Table 2, the amounts ofNMP, ethanol, PPD, and MPD were the same as are set forth in Table 1 forthe samples prepared, the PPD/MPD diamine component mix thus being inthe ratio of PPD:MPD: :95:5.

Table 3 sets forth the characteristics measured for the polyimidepolymers made from each sample mixture shown in Table 2, including thecomparative sample of NR-150 prepared. For each sample, the density ofthe polyimide is provided in grams per cubic centimeter (Density,gms/cc) and the glass transition temperature as measured by DynamicMechanical analysis is given in degrees Centigrade (Tg° C). The thermaloxidative stability (TOS) for the samples and the standard is given inthe table as the percent weight loss determined as set forth above, thatis after 100 hours in air at 750° F. (399° C.). It should be noted that,measured in this way, higher thermal oxidative stability is shown by alower percent weight loss. The moisture regain values in Table 3(Moisture Regain) were also measured as set forth above, that is, aspercent weight gain after 31 days at 60° C. and 95% relative humidity.The values for Moisture Regain are in percent weight gain. It should benoted with respect to the data given in Table 3 that the NR-150 wascured under the same conditions as the samples made from the BTDA andBPDA mixes to normalize conditions.

Table 3 amply illustrates the advantages of the polyimides of thecurrent invention over those known in the art. The respective glasstransition temperatures of the resins, particularly those made withBTDA/BPDA ratios ranging from about BTDA:BPDA: :40:60 to about 80:20compare favorably with the Tg shown for NR-150. Thus these resins,particularly the 70:30 resin, can be advantageously used in makingcomposites for high temperature applications. The TOS of each sample,measured by percent weight loss, is greater in each instance than thatof NR-150, that is, the percent weight losses of the samples of thecurrent invention are lower than the percent weight loss measured forNR-150, and the TOS of the 70:30 resin is significantly better than thatof the NR-150 TOS, the disclosed 70:30 suffering a weight loss of only4.3% compared to the weight loss for NR-150 of 10.7%. Moreover, themeasured moisture regain, measured as percent weight gain at saturation,for each sample resin is better than that for NR-150, the 70:30 resinagain showing a moisture regain value of less than half of the NR-150control.

The novel polyimides made in accordance with the current invention showsuperior TOS and moisture regain characteristics, while at the same timehaving Tg characteristics which make them useful in a wide variety ofapplications.

The characteristics of the polyimides of the current invention areunexpected. Those of ordinary skill in this art, considering thecompounds 6FTA, BPDA, and BTDA used as acid functional components inpolyimides, would generally expect that as the ratio of BTDA present isincreased in polyimides, the resulting polyimide would exhibit lowerthermal oxidative stability and a higher moisture regain characteristic.It has been reported, for example, that the thermal oxidative stabilityof BPDA should be greater than that of BTDA. See Bessonov et al.,Polyimides: Thermally Stable Polymers, trans. ed. W. W. Wright, pp.103-108 (Consultants Bureau, N.Y., N.Y. 1987). It is also unexpectedthat a polyimide made using BTDA and BPDA as the acid functionalcomponents would have better TOS and moisture regain characteristicsthan those exhibited by NR-150. Contrary to these expectations, as shownby the polyimides of the current invention, there is a range in whichincreasing the ratio of BTDA to BPDA in the acid functional componentresults in a polyimide having superior values for the moisture regainand thermal oxidative stability characteristics.

The components used in making the disclosed polyimides are readilyavailable and economical. This, and the desirable characteristics of thepolyimides of the current invention as shown herein, will make use ofthese polyimides advantageous in a number of applications. The high Tgand TOS characteristics provide excellent mechanical properties, makingthe polyimides of the current invention useful in applications requiringresin alone or as a component of composite structures. These and theother properties also make use of the resins of the current inventionadvantageous in the form of reinforced or unreinforced films andlaminates. Products made utilizing the resins of the current inventionare expected to have a relatively long life in applications whereexposure to water is expected because of the low moisture regaincharacteristics as compared to other polyimides having relatively highermoisture regain properties.

The polyimides of the current invention may also be advantageously usedas the resin matrices for composites. The high Tg should enable use inhigh-temperature applications such as machine or engine parts, and theTOS and moisture regain properties should contribute to providing alonger life-span for such composites. The substrates for which theresins of the current invention can be used as the polymer matrix can beany of those known in the art, including but not limited to fibroussubstrates, metals, carbon or glass fibers or particles, and aramidfibers.

                  TABLE 1                                                         ______________________________________                                        RECIPE FOR 70/30 MOLE % BTDA/BPDA AND 95/5 MOLE %                               PPD/MPD POLYIMIDE SOLUTION                                                         INGREDIENT     MOLES   GMS                                             ______________________________________                                        NMP               --      128                                                   ETHANOL -- 128                                                                BTDA 0.369 118.8                                                              BPDA 0.158 46.5                                                               PPD 0.499 54.0                                                                MPD 0.027 30                                                                ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        RECIPE FOR BTDA/BPDA POLYIMIDE SOLUTIONS                                                   Mole Percent      Weight (gms)                                   Sample       BTDA    BPDA      BTDA  BPDA                                     ______________________________________                                        1            80      20        136.0 31.0                                       2 75 25 127.3 39.0                                                            3 70 30 118.8 46.5                                                            4 60 40 101.8 62.4                                                            5 40 60 67.9 93.6                                                             6 30 70 50.9 109.2                                                            7 20 80 33.9 124.8                                                          Control (NR-150)                                                                           100 (6FTA)    253 (6FTA)                                         ______________________________________                                         Note: The amounts of NMP, ethanol, PPD, and MPD are the same as in Table      1.                                                                       

                  TABLE 3                                                         ______________________________________                                        BTDA/BPDA POLYMERS AND THEIR PROPERTIES                                                   BTDA/BPDA  DENSITY                                                                              Tg         Moisture                               Sample RATIO Gms/cc ° C. TOS Regain                                  ______________________________________                                        1       80/20      1.42     345   6.5  1.8                                      3 70/30 1.43 338 4.3 1.1                                                      4 60/40 1.37 317 -- 1.2                                                       5 40/60 1.43 302 2.1 0.5                                                      6 30/70 1.43 299 2.4 0.6                                                      7 20/80 1.43 305 2.0 1.1                                                      Control 100(6FTA) 1.45 379 10.7  2.4                                          (NR-150)                                                                    ______________________________________                                         Note: The diamine component for all of the above was PPD:MPD::95:5 by mol     percent                                                                  

We claim:
 1. A polyimide polymer having a glass transition temperatureof greater than about 300 degrees Centigrade comprising structural unitsderived from 3,4,3',4'-benzophenonetetracarboxylic dianhydride,3,4,3',4'-biphenyltetracarboxylic dianhydride and one or more aromaticdiamines, wherein the mole ratio of the structural units derived from3,4,3',4'-benzophenonetetracarboxylic dianhydride to the structuralunits derived from 3,4,3',4'-biphenyltetracarboxylic dianhydride is inthe range of from more than about 3.0/7.0 and up to about 4.0/1.0.
 2. Apolyimide polymer according to claim 1 wherein the aromatic diamines areselected from the group comprising m-phenylenediamine,p-phenylenediamine, 1,4-bis(4-aminophenoxy)-2-phenylbenzene, andmixtures thereof.
 3. A process of preparing a polyimide polymer having aglass transition temperature of greater than about 300 degreesCentigrade comprising:a. forming a precursor solution by preparing asolution of an acid functional component containing3,4,3',4'-benzophenonetetracarboxylic dianhydride and3,4,3',4'-biphenyltetracarboxylic dianhydride in a polar aprotic solventto form the diethylester diacid forms of the dianhydrides, wherein themole ratio of the structural units derived from3,4,3',4'-benzophenonetetracarboxylic dianhydride to the structuralunits derived from 3,4,3',4'-biphenyltetracarboxylic dianhydride is inthe range of from more than about 3.0/7.0 and up to about 4.0/1.0 andreacting with a diamine component containing one or more aromaticdiamines and b. heating the precursor solution to devolatilize andpolymerize the precursor solution to the polyimide.
 4. The process ofclaim 3 wherein either the acid functional component or the diaminecomponent contains an end capping agent and there is a stoichiometricexcess of either component with respect to the other of up to about 40mole percent.
 5. A polyimide polymer according to claim 1, wherein themole ratio of the structural units derived from3,4,3',4'-benzophenonetetracarboxylic dianhydride to the structuralunits derived from 3,4,3',4'-biphenyltetracarboxylic dianhydride is inthe range of from about 3.0/2.0 to 4.0/1.0.
 6. A polyimide polymeraccording to claim 1, wherein the mole ratio of the structural unitsderived from 3,4,3',4'-benzophenonetetracarboxylic dianhydride to thestructural units derived from 3,4,3',4'-biphenyltetracarboxylicdianhydride is about 7.0/3.0.
 7. A composite comprising the polyimidepolymer of claim 1 reinforced with a substrate.
 8. A precursorcomposition for preparing a solution containing the polyimide polymer ofclaim 1 comprising as the acid functional component3,4,3',4'-benzophenonetetracarboxylic dianhydride and3,4,3',4'-biphenyltetracarboxylic dianhydride and as the diaminecomponent aromatic diamines, wherein the mole ratio of the structuralunits derived from 3,4,3',4'-benzophenonetetracarboxylic dianhydride tothe structural units derived from 3,4,3',4'-biphenyltetracarboxylicdianhydride is in the range of from more than about 3.0/7.0 and up toabout 4.0/1.0.
 9. A prepreg comprising a fibrous substrate impregnatedwith the precursor composition of claim
 8. 10. A polyimide polymercomprising an acid functional component containing structural unitsderived from 3,4,3',4'-benzophenonetetracarboxylic dianhydride and3,4,3',4'-biphenyltetracarboxlyic dianhydride wherein the mole ratio ofthe structural units derived from 3,4,3',4'-benzophenonetetracarboxylicdianhydride to the structural units derived from3,4,3',4'-biphenyltetracarboxylic dianhydride is in the range of frommore than about 3.0/7.0 and up to about 4.0/1.0; a diamine componentcontaining one or more aromatic diamines; and an end capping agent. 11.A polyimide having a glass transition temperature of about 338 degreesCentigrade, a thermal oxidative stability of about 4.3 percent, and amoisture regain of about 1.1 percent comprising; an acid functionalcomponent of 70 mole percent 3,4,3',4'-benzophenonetetracarboxylicdianhydride and 30 mole percent 3,4,3',4'-biphenyltetracarboxylicdianhydride, and a diamine component of from about 95 to 100 molepercent p-phenylenediamine and from 0 to about 5 mole percentm-phenylenediammne.
 12. A film comprising the polyimide polymer ofclaim
 1. 13. A laminate comprising the polyimide polymer of claim 1.